The present invention relates to the field of communications and more particularly to communications methods and receivers.
Due to the limited availability of the signal spectrum, cellular radiotelephone systems have been developed wherein carrier frequencies are re-used in distant cells to increase spectral efficiency. Because of this frequency reuse, however, co-channel interference may be present at both mobile terminals and base stations. In response, there have been efforts to develop signal enhancing receivers to reduce the effects of co-channel interference. For example, see the reference by Medepalli et al. entitled xe2x80x9cCombined Equalization And CoChannel Interference Cancellation For The Downlink Using Tentative Decisionsxe2x80x9d (IEEE 1999) the disclosure of which is hereby incorporated herein in its entirety by reference.
The effects of co-channel interference (CCI) can conventionally be reduced by providing signal separation in the transmission of different signals. Cochannel signal separation is conventionally provided in an FDMA system by providing physical separation between two transmitters using the same carrier frequency and between the respective receiving base stations. Accordingly, a first signal is received by the first base station at a significantly higher strength than a second signal, and the second signal is received by the second base station at a significantly higher strength than the first signal. As cell sizes are reduced to provide greater capacity, however, the differences in signal strengths may be reduced making it difficult to receive one or both co-channel signals. Interference from signals transmitted on adjacent carrier frequencies (adjacent channel interference or ACI) can be accommodated by filtering the carrier frequency of interest.
In CDMA systems, signal separation is provided during transmission by correlating a first signal with a first spreading code and correlating the second signal with a second spreading code. The two signals can then be separated by decorrelating the desired signal with the respective spreading code. In other words, channel separation is provided during transmission through the use of different spreading codes.
The reception of co-channel signals without prior signal channel separation during transmission, however, may be difficult. Accordingly, there continues to exist a need for methods and receivers that can receive multiple information signals over a common carrier frequency.
It is therefore an object of the present invention to provide improved methods for receiving information signals and related receivers.
It is another object of the present invention to provide methods and receivers that can receive multiple information signals over a common carrier frequency wherein the multiple information signals are transmitted on the common carrier frequency without significant channel separation.
It is still another object of the present invention to provide methods and receivers that can receive multiple information signals over a common carrier frequency wherein the multiple information signals have common signal strengths.
These and other objects can be provided according to the present invention by providing some degree of signal separation at the receiver followed by successive cancellation. The signal separation, for example, can be provided by using spatial and/or temporal processing of co-channel signals, by using cyclostationary properties of the co-channel signals, or by using coding and decoding.
According to a first aspect of the present invention, a plurality of communications are received from a respective plurality of transmitters using a common carrier frequency. In particular, a plurality of information signals are received on a common carrier frequency corresponding to the plurality of communications from the plurality of transmitters. First and second separated baseband signals are generated corresponding to respective first and second ones of the information signals so that the first separated baseband signal includes a primary component corresponding to the first information signal and so that the second separated baseband signal includes a primary component corresponding to the second information signal. The first separated baseband signal is demodulated to provide an estimate of a first information sequence corresponding to the first information signal. The estimate of the first information sequence is modulated to provide a modulated estimate of the first information sequence, and the modulated estimate of the first information sequence is subtracted from the second separated baseband signal to provide an improved second separated baseband signal. The improved second separated baseband signal is then demodulated to provide an estimate of a second information sequence corresponding to the second information signal.
Moreover, the estimates of the first and second information sequences can be processed to reproduce the first and second communications. By providing the separated baseband signals to have at least some separation, successive cancellation can be used to further improve reception and reproduction of the communications. In addition, the step of modulating the estimate of the first information sequence is preceded by decoding the estimate of the first information sequence to improve the estimate of the first information sequence, and re-encoding the improved estimate of the first information sequence so that the improved and re-encoded estimate of the first information sequence is modulated during the modulating step. The decoding step thus provides a higher degree of signal separation before successive cancellation. The step of subtracting the modulated estimate can also include extrapolating the modulated estimate of the first information sequence to estimate a component of the second separated baseband signal attributable to the first information signal.
More particularly, the step of receiving the plurality of information signals can include receiving first and second co-channel signals from respective first and second antennas. The step of generating the first and second separated baseband signals can thus include combining the first and second co-channel signals according to a first combining branch to generate the first separated baseband signal corresponding to the first information signal, and combining the first and second co-channel signals according to a second combining branch to generate the second separated baseband signal corresponding to the second information signal.
Alternately, the step of receiving the plurality of information signals can include receiving a co-channel signal. In addition, the step of generating the first and second separated baseband signals can include filtering the co-channel signal using a first filter to generate the first separated baseband signal corresponding to the first information signal, and filtering the co-channel signal using a second filter different from the first filter to generate the second separated baseband signal corresponding to the second information signal. More particularly, the first filter can include a first medium response estimate for the first information signal and the second filter can include a second medium response estimate for the second information signal.
According to yet another alternative, the step of receiving the plurality of information signals can include receiving a co-channel signal. In this alternative, the step of generating the first and second separated baseband signals includes sampling the co-channel signal with a first sampling phase to generate a first sampled co-channel signal, sampling the co-channel signal with a second sampling phase different than the first sampling phase to generate a second sampled co-channel signal, combining the first and second sampled co-channel signals using a first filter combination to provide the first separated baseband signal, and combining the first and second sampled co-channel signals using a second filter combination to provide the second separated baseband signal.
According to still another alternative, the step of receiving the plurality of information signals can include receiving a co-channel signal. Here, the step of generating the first and second separated baseband signals includes sampling the co-channel signal with a first sampling phase to generate the first separated baseband signal, and sampling the co-channel signal with a second sampling phase different than the first sampling phase to generate the second separated baseband signal. The method can also include modulating the estimate of the second information sequence to provide a modulated estimate of the second information sequence, subtracting the modulated estimate of the second communication from the first separated baseband signal to provide an improved first separated baseband signal, and demodulating the improved first separated baseband signal to provide an improved estimate of the first information sequence. Additional levels of successive cancellation can thus be provided.
According to another aspect of the present invention, a plurality of communications can be received from a respective plurality of transmitters using a common carrier frequency by receiving a plurality of information signals on a common carrier frequency corresponding to the plurality of communications from the plurality of transmitters, and generating a co-channel baseband signal responsive to the plurality of information signals. The co-channel baseband signal is demodulated to provide an estimate of a first information sequence corresponding to a first one of the communications, the estimate of the first information sequence is decoded to improve the estimate of the first information sequence, and the improved estimate of the first information sequence is re-encoded. The improved and re-encoded estimate of the first information sequence is modulated and subtracted from the co-channel baseband signal to provide a second baseband signal, and the second baseband signal is demodulated to provide an estimate of a second information sequence corresponding to a second one of the communications. Decoding and re-encoding can thus provide signal separation for two information signals transmitted on a common carrier frequency.
Methods and receivers according to the present invention thus allow improved reception of multiple information signals transmitted without channel separation over a common carrier frequency.